Method for converting a standalone network storage system into a disk drive storage enclosure

ABSTRACT

An existing disk drive storage enclosure is converted into a standalone network storage system by removing one or more input/output (I/O) modules from the enclosure and installing in place thereof one or more server modules (“heads”), each implemented on a single circuit board. Each head contains the electronics, firmware and software along with built-in I/O connections to allow the disks in the enclosure to be used as a Network-Attached file Server (NAS) or a Storage Area Network (SAN) storage device. An end user can also remove the built-in head and replace it with a standard I/O module to convert the enclosure back into a standard disk drive storage enclosure. Two internal heads can communicate over a passive backplane in the enclosure to provide full cluster failover (CFO) capability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/407,681, filed on Apr. 4, 2003, now U.S. Pat. No. 7,127,798, andentitled, “Method for Converting Disk Drive Storage Enclosure into aStandalone Network Storage System”. In addition, this application isrelated to U.S. patent application Ser. No. 10/407,535, filed on Apr. 4,2003 and entitled, “Standalone Network Storage System EnclosureIncluding Head and Multiple Disk Drives Connected to a PassiveBackplane”, and to U.S. patent application Ser. No. 10/407,538, filed onApr. 4, 2003 and entitled, “Standalone Storage System with MultipleHeads in an Enclosure Providing Cluster Failover Capability”.

FIELD OF THE INVENTION

At least one embodiment of the present invention pertains to storagesystems, and more particularly, to a method and apparatus for convertinga disk drive storage enclosure into a standalone network storage systemand vice versa.

BACKGROUND

A file server is a network-connected processing system that stores andmanages shared files in a set of storage devices (e.g., disk drives) onbehalf of one or more clients. The disks within a file system aretypically organized as one or more groups of Redundant Array ofIndependent/Inexpensive Disks (RAID). One configuration in which fileservers can be used is a network attached storage (NAS) configuration.In a NAS configuration, a file server can be implemented in the form ofan appliance that attaches to a network, such as a local area network(LAN) or a corporate intranet. An example of such an appliance is any ofthe Filer products made by Network Appliance, Inc. in Sunnyvale, Calif.

Another specialized type of network is a storage area network (SAN). ASAN is a highly efficient network of interconnected, shared storagedevices. Such devices are also made by Network Appliance, Inc. Onedifference between NAS and SAN is that in a SAN, the storage applianceprovides a remote host with block-level access to stored data, whereasin a NAS configuration, the file server normally provides clients withonly file-level access to stored data.

Current file server systems used in NAS environments are generallypackaged in either of two main forms: 1) an all-in-one custom-designedsystem that is essentially just a standard computer with built-in diskdrives, all in a single chassis (enclosure); or 2) a modular system inwhich one or more sets of disk drives, each in a separate chassis, areconnected to an external file server “head” in another chassis. Examplesof all-in-one file server systems are the F8x, C1xxx and C2xxx seriesFilers made by Network Appliance of Sunnyvale, Calif.

In this context, a “head” means all of the electronics, firmware and/orsoftware (the “intelligence”) that is used to control access to storagedevices in a storage system; it does not include the disk drivesthemselves. In a file server, the head normally is where all of the“intelligence” of the file server resides. Note that a “head” in thiscontext is not the same as, and is not to be confused with, the magneticor optical head used to physically read or write data to a disk.

In a modular file server system, the system can be built up by addingmultiple chassis in some form of rack and then cabling the chassistogether. The disk drive enclosures are often called “shelves” and, morespecifically, “just a bunch of disks” (JBOD) shelves. The term JBODindicates that the enclosure essentially contains only physical storagedevices and no electronic “intelligence”. Some disk drive enclosuresinclude one or more RAID controllers, but such enclosures are notnormally referred to as “JBOD” due to their greater functionalcapabilities. A modular file server system is illustrated in FIG. 1 andis sometimes called a “rack and stack” system. In FIG. 1, a file serverhead 1 is connected by external cables to multiple disk shelves 2mounted in a rack 3. The file server head 1 enables access to storeddata by one or more remote client computers (not shown) that areconnected to the head 1 by external cables. Examples of modular headssuch as head 1 in FIG. 1 are the FAS800 and FAS900 series filer headsmade by Network Appliance.

A problem with the all-in-one type of system is that it is not veryscalable. In order to upgrade the server head, the user needs to swapout the old system and bring in a new one, and then he has to physicallymove drives from the old enclosure to the new enclosure. Alternatively,the user could copy data from the old to the new, however, doing sorequires double the disk capacity during the copy operation (one set tohold the old source data and one set to hold the new data) and anon-trivial amount of time to do the copying. Neither of theseapproaches is simple or easy for a user to do.

A problem with the modular type of system is that it is notcost-effective for smaller, minimally-configured storage systems. Thereis a fixed overhead of at least two chassis (one head plus one diskshelf) with their power supplies and cooling modules as well asadministrative overhead associated with cabling one chassis to the otherand attendant failures associated with cables. In order to make eachhead as modular as possible, the head itself typically includes amotherboard and one or more input/output (I/O) boards. Theinfrastructure to create this modularity is amortized across the fullyconfigured systems but represents high overhead for minimally configuredsystems.

What is needed, therefore, is a network storage system which overcomesthese disadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 illustrates a portion of a “rack and stack” (modular) file serversystem;

FIG. 2 is a block diagram of a modular file server system;

FIG. 3 illustrates in greater detail a disk drive shelf of the fileserver system of FIG. 2;

FIG. 4 is an architectural block diagram of a file server head;

FIG. 5 is a hardware layout block diagram of a JBOD disk drive shelf;

FIG. 6 is a perspective diagram showing the internal structure of a JBODdisk drive shelf being converted into a standalone storage server;

FIG. 7 is a hardware layout block diagram of a standalone file serverconstructed from a JBOD disk drive shelf;

FIG. 8 illustrates a standalone file server, with a file server headimplemented on a single circuit board connected to a passive backplane;and

FIG. 9 is a block diagram of a single-board head.

DETAILED DESCRIPTION

A method and apparatus for converting a JBOD disk drive storageenclosure into a standalone network storage system and vice versa aredescribed. Note that in this description, references to “one embodiment”or “an embodiment” mean that the feature being referred to is includedin at least one embodiment of the present invention. Further, separatereferences to “one embodiment” or “an embodiment” in this description donot necessarily refer to the same embodiment; however, such embodimentsare also not mutually exclusive unless so stated, and except as will bereadily apparent to those skilled in the art from the description. Forexample, a feature, structure, act, etc. described in one embodiment mayalso be included in other embodiments. Thus, the present invention caninclude a variety of combinations and/or integrations of the embodimentsdescribed herein.

As described in greater detail below, a JBOD disk drive shelf can beconverted into a standalone network storage system by removing one ormore input/output (I/O) modules from its enclosure and installing inplace of the I/O modules one or more heads, each implemented on a singlecircuit board. Each such head contains the electronics, firmware andsoftware along with built-in I/O connections to allow the disks in theenclosure to be used as a NAS file server and/or a SAN storage device.Two internal heads can communicate over a passive backplane in theenclosure to provide full cluster failover (CFO) capability. An end usercan also remove the built-in head and replace it with a standard I/Omodule to convert the enclosure back into a standard JBOD disk drivestorage enclosure. This standard enclosure could then be grown incapacity and/or performance by combining it with additional modularstorage shelves and a separate, more-capable modular file server head.This approach provides scalability and upgradability with minimum effortrequired by the user.

FIG. 2 is a functional block diagram of a modular type file serversystem such as mentioned above. A modular file server head 1 iscontained within its own enclosure and is connected to a number of theexternal disk drive shelves 2 in a loop configuration. Each shelf 2contains multiple disk drives 23 operated under control of the head 1according to RAID protocols. The file server head 1 provides a number ofclients 24 with access to shared files stored in the disk drives 23.Note that FIG. 2 shows a simple network configuration characterized by asingle loop with three shelves 2 in it; however, other networkconfigurations are possible. For example, there can be a greater orsmaller number of shelves 2 in the loop; there can be more than one loopattached to the head 1; or, there can even be one loop for every shelf2.

FIG. 3 illustrates in greater detail a disk drive shelf 2 of the typeshown in FIGS. 1 and 2 (the clients 24 are not shown). Each of theshelves 2 can be assumed to have the same construction. Each shelf 2includes multiple disk drives 23. Each shelf also includes at least oneI/O module 31, which is connected between the shelf 2 and the next shelf2 in the loop and in some cases (depending on where the shelf 2 isplaced in the loop) to the head 1. The I/O module 31 is a communicationsinterface between the head 1 and the disk drives 23 in the shelf 2. Thefunctionality of the I/O module 31 is described further below. The diskdrives 23 in the shelf 2 can be connected to the I/O module 31 by astandard Fibre Channel connection.

The use of RAID protocols between the head 1 and the shelves 2 enhancesthe reliability/integrity of data storage through the redundant writingof data “stripes” across physical disks 23 in a RAID group and theappropriate writing of parity information with respect to the stripeddata. In addition to acting as a communications interface between thehead 1 and the disk drives 23, the I/O module 31 also serves to enhancereliability by providing loop resiliency. Specifically, if a particulardisk drive 23 within a shelf 2 is removed, the I/O module 31 in thatshelf 2 simply bypasses the missing disk drive and connects to the nextdisk drive within the shelf 2. This functionality maintains connectivityof the loop in the presence of disk drive removals and is provided bymultiple Loop Resiliency Circuits (LRCs) (not shown) included within theI/O module 31. In at least one embodiment, the LRCs are implemented inthe form of port bypass circuits (PBCs) within the I/O module 31(typically, a separate PBC for each disk drive 23 in the shelf 2). Notethat a PBC is only one (simple) implementation of an LRC. Other ways toimplement an LRC include a hub or a switch, although these approachestend to be more complicated. The implementation details of I/O modulesand PBCs such as described here are well known in the relevant art andare not needed to understand the present invention.

As mentioned above, access to data in a file server system is controlledby a file server head, such as head 1 in the above-described figures.Also as described above, in a modular file server system the head 1 iscontained within its own chassis and is connected to one or moreexternal JBOD disk shelves 2 in their own respective chassis. FIG. 4 isan architectural block diagram of such a file server head 1, accordingto certain embodiments. As shown, the head 1 includes a processor 41,memory 42, and a chipset 43 connecting the processor 41 to the memory42. The chipset 43 also connects a peripheral bus 44 to the processor 41and memory 42. Also connected to the peripheral bus 44 are one or morenetwork adapters 45, one or more storage adapters 46, one or moremiscellaneous I/O components 47, and in some embodiments, one or moreother peripheral components 48. The head 1 also includes one or morepower supplies 49 and one or more cooling modules 50 (preferably atleast two of each for redundancy).

The processor 41 is the central processing unit (CPU) of the head 1 andmay be, or may include, one or more programmable general-purpose orspecial-purpose microprocessors, digital signal processors (DSPs),programmable controllers, application specific integrated circuits(ASICs), programmable logic devices (PLDs), or the like, or acombination of such devices. The memory 42 may be or include anycombination of random access memory (RAM), read-only memory (ROM) (whichmay be programmable) and/or Flash memory or the like. The chipset 43 mayinclude, for example, one or more bus controllers, bridges and/oradapters. The peripheral bus 44 may be, for example, a PeripheralComponent Interconnect (PCI) bus, a HyperTransport or industry standardarchitecture (ISA) bus, a small computer system interface (SCSI) bus, auniversal serial bus (USB), or an Institute of Electrical andElectronics Engineers (IEEE) standard 1394 bus (sometimes referred to as“Firewire”). Each network adapter 45 provides the head 1 with theability to communicate with remote devices, such as clients 24 in FIG.2, and may be, for example, an Ethernet adapter. Each storage adapter 46allows the head 1 to access the external disk drives 23 in the variousshelves 2 and may be, for example, a Fibre Channel adapter.

FIG. 5 is a hardware layout block diagram of a JBOD disk drive shelf 2of the type which may be connected to a separate (external) head 1 in amodular file server system. All of the illustrated components arecontained within a single chassis. As shown, all of the major componentsof the shelf 2 are connected to, and communicate via, a passivebackplane 51. The backplane 51 is “passive” in that it has no activeelectronic circuitry mounted on or in it; it is just a passivecommunications medium. The backplane 51 can be essentially comprised ofjust one or more substantially planar substrate layers (which may beconductive or which may be dielectric with conductive traces disposedon/in it), with various pin-and-socket type connectors mounted on it toallow connection to other components in the shelf 2.

Connected to the backplane 51 in the shelf 2 of FIG. 5 are severalindividual disk drives 23, redundant power supplies 52 and associatedcooling modules 53 (which may be substantially similar to power supplies49 and cooling modules 50, respectively, in FIG. 4), and two I/O modules31 of the type described above. As described above, the I/O modules 31provide a communications interface between an external head 1 and thedisk drives 23, including providing loop resiliency for purposes ofaccessing the disk drives 23.

In accordance with at least one embodiment of the invention, a JBOD diskdrive shelf 2 such as shown in FIG. 5 can be converted into a standalonenetwork storage system by removing the I/O modules 31 from the chassisand installing in place of them one or more server heads, eachimplemented on a separate, single circuit board (hereinafter“single-board heads”). Each single-board head contains the electronics,firmware and software along with built-in I/O connections to allow theenclosure to be used as a NAS file server and/or a SAN storage system.The circuit board of each single-board head has various conventionalelectronic components (processor, memory, communication interfaces,etc.) mounted and interconnected on it, as described in detail below. Inother embodiments, the head can be distributed between two or morecircuit boards, although a single-board head is believed to beadvantageous from the perspective of conserving space inside thechassis.

FIG. 6 shows the interior of the JBOD shelf 2 of FIG. 5, as it is beingconverted into a standalone storage system (e.g., a NAS file serverand/or a SAN storage system), in accordance with at least one embodimentof the invention. The chassis 61 of the shelf 2 is shown transparent tofacilitate illustration. In the illustrated embodiment, the passivebackplane 51 is mounted within the chassis 61 so as to divide thechassis 61 roughly in half, so as to define a front portion 62 of thechassis 61 from a rear portion 63 of the chassis. To facilitateillustration, the disk drives 23 are not shown in FIG. 6, although inthe illustrated embodiment they would normally be stacked side-by-sidein the front portion 62 of the chassis 61 and connected to the backplane51. Installed against each outer edge of the rear portion 63 of thechassis 61 are the two power supplies 52 and their cooling modules (notshown). The two I/O modules 31 are normally stacked on top of each otherbetween the two power supplies 52 in the center of the rear portion 63of the chassis 61 and are normally connected to the backplane 51.Examples of JBOD storage shelves that have a construction similar tothat shown in FIGS. 5 and 6 are the RS-1600-FC, SS-1201-FC andSS-1202-FC storage enclosures made by Xyratex, Ltd. of Havant, UnitedKingdom.

To convert the JBOD shelf 2 into a standalone storage system, the I/Omodules 31 are disconnected from the backplane 51, removed from theenclosure, and replaced with one or more single-board heads 64, asshown. The single-board head or heads 64 are connected to the passivebackplane 51. The area or “footprint” of each single-board head 64 is nolarger than the combined footprint of the stacked I/O modules 31. If twoor more single-board heads 64 are installed, they are stacked on top ofeach other within the chassis 61.

FIG. 7 is a hardware layout block diagram of a standalone storage system71 after its conversion from a JBOD shelf 2 as described above. Theblock diagram is substantially the same as that of FIG. 5, except thateach of the I/O modules 31 has been replaced by a single-board head 64connected to the passive backplane 51. Connecting the heads 64 to thebackplane 51 is advantageous, because, among other reasons, iteliminates the need for cables or wires to connect the heads 64. Notethat although two heads 64 are shown in FIG. 7, the device 71 canoperate as a standalone system with only one head 64.

This standalone system 71 can be easily grown in capacity and/orperformance by combining it with additional modular storage shelves and(optionally) with a separate, more capable file server head. Thisapproach provides scalability and upgradability with minimum effortrequired by the user. In addition, this approach allows the user to addmore performance or capacity to his system without physically movingdisk drives from the original enclosure or having to copy the data fromthe original machine to the newer, more capable machine.

FIG. 8 shows a rear perspective view of the standalone storage system 71according to at least one embodiment of the invention, with onesingle-board head 64 installed. Not shown in FIG. 8 are the disk drives23, which are normally installed against the far side of the backplane51. The single-board head 64 includes various electronic componentsmounted on a circuit board 80 that is connected to the backplane 51between the two power supplies 52. The single-board head 64 is connectedto the backplane 51 by a number of conventional pin-and-socket typeconnector pairs 81 mounted on the circuit board and the backplane 51,which may be, for example, connectors with part nos. HM1L52ZDP411H6P and84688-101 from FCI Electronics/Burndy or similar connectors from TycoElectronics/AMP.

This manner of installation also allows the single-board head or heads64 to be easily disconnected and removed, and I/O modules 31 installed(or reinstalled) in place thereof, to convert the system into (or backinto) a JBOD shelf. In that case, the JBOD shelf can then be attachedwith stored data intact to a larger, more capable head (possibly withadditional shelves). As noted, this allows the user to add moreperformance or capacity to his system without physically moving drivesfrom the original shelf or having to copy the data from the originalmachine to the newer, more capable machine.

FIG. 9 is a block diagram of a single-board head 64, according tocertain embodiments of the invention. The single-board head 64 includes(mounted on a single circuit board 80) a processor 91, dynamic read-onlymemory (DRAM) 92 in the form of one or more dual inline memory modules(DIMMs), an integrated circuit (IC) Fibre Channel adapter 93, and anumber of Fibre Channel based (IC) PBCs 94. The processor 91 controlsthe operation of the head 64 and, in certain embodiments, is a BCM1250multi-processor made by Broadcom Corp. of Irvine, Calif. The DRAM 92serves as the main memory of the head 64, used by the processor 91.

The PBCs 94 are connected to the processor 91 through the Fibre Channeladapter 93 and are connected to the passive backplane 51 throughstandard pin-and-socket type connectors 81 (see FIG. 8) mounted on thecircuit board 81 and on the backplane 51, such as described above. ThePBCs 94 are connected to the Fibre Channel adapter 93 in a loopconfiguration, as shown in FIG. 9. In operation, each PBC 94 cancommunicate (through the backplane 51) separately with two or more diskdrives installed within the same chassis. Normally, each PBC 94 isresponsible for a different subset of the disk drives within thechassis. Each PBC 94 provides loop resiliency with respect to the diskdrives for which it is responsible, to protect against a disk drivefailure in essentially the same manner as done by the I/O modules 31described above. In other words, in the event a disk drive fails, theassociated PBC 94 will simply bypass the failed disk drive. Examples ofPBCs with such functionality are the HDMP-0480 and HDMP-0452 fromAgilent Technologies in Palo Alto, Calif., and the VSC7127 from VitesseSemiconductor Corporation in Camarillo, Calif.

The single-board head 64 also includes (mounted on the circuit board 80)a number of IC Ethernet adapters 95. In the illustrated embodiment, twoof the Ethernet adapters 95 have external connectors to allow them to beconnected to devices outside the chassis for network communication(e.g., to clients); the third Ethernet adapter 95A is routed only to oneof the connectors 81 (shown in FIG. 8) that connects to the backplane51. The third Ethernet adapter 95A (which is connectable to thebackplane 51) can be used to communicate with another single-board head64 installed within the same chassis, as described further below.

The single-board head 64 further includes (mounted on the circuit board80) a standard RJ-45 connector 96 which is coupled to the processor 91through a standard RS-232 transceiver 97. This connector-transceiverpair 96 and 97 allows an external terminal operated by a networkadministrator to be connected to the head 64, for purposes of remotelymonitoring or configuring the head 64 or other administrative purposes.

The single-board head 64 also includes (mounted on the circuit board 80)at least one non-volatile memory 98 (e.g., Flash memory), which storesinformation such as boot firmware, a boot image, test software and thelike. The single-board head 64 also includes (mounted on the circuitboard 80) a connector 99 to allow testing of the single-board head 64 inaccordance with JTAG (IEEE 1149.1) protocols.

The single-board head 64 shown in FIG. 9 also includes (mounted on thecircuit board 80) two Fibre Channel connectors 102 to allow connectionof the head 64 to external components. One of the Fibre Channelconnectors 102 is coupled directly to the Fibre Channel adapter 93,while the other Fibre Channel connector 102A is coupled to the FibreChannel adapter 93 through one of the PBCs 94. Fibre Channel connector102A can be used to connect the head 64 to an external disk shelf.Although the single-board head 64 allows the enclosure to be used as astandalone file server without any external disk drives, it maynonetheless be desirable in some cases to connect one or more externalshelves to the enclosure to provide additional storage capacity.

In certain embodiments, the processor 91 in the single-board head 64 isprogrammed (by instructions and data stored in memory 92 and/or inmemory 98) so that the enclosure is operable as both a NAS file server(using file-level accesses to stored data) and a SAN storage system(using block-level accesses to stored data) at the same time, i.e., tooperate as a “unified” storage device, sometimes referred to as fabricattached storage (FAS) device. In other embodiments, the single-boardhead 64 is programmed so that the enclosure is operable as either a NASfile server or a SAN storage, but not at the same time, where the modeof operation can be determined after deployment according to a selectionby a user (e.g., a network administrator). In other embodiments of theinvention, the single-board head 64 is programmed so that the enclosurecan operate only as a NAS file server or, in still other embodiments,only as a SAN storage system.

If the single-board head is configured to operate as a NAS fileserver,the single-board head 64 can be configured with the ability to usemultiple file based protocols. For example, in certain embodiments thesingle-board head 64 is able to use each of network file system (NFS),common Internet file system (CIFS) and hypertext transport protocol(HTTP), as necessary, to communicate with external devices, such as diskdrives and clients.

As noted above, two or more single-board heads 64 can be included in thestandalone system. The inclusion of two or more heads 64 enables thestandalone system to be provided with cluster failover (CFO) capability(i.e., redundancy), while avoiding much of the cost and spaceconsumption associated with providing CFO in prior art systems. CFOrefers to a capability in which two or more interconnected heads areboth active at the same time, such that if one head fails or is takenout of service, that condition is immediately detected by the otherhead, which automatically assumes the functionality of the inoperativehead as well as continuing to service its own client requests. A fileserver “cluster” is defined to include at least two file server headsconnected to at least two separate volumes of disks. In known prior artmodular file server systems, a “cluster” includes at least two diskshelves and at least two heads in separate enclosures; thus, at leastfour separate chassis are needed to provide CFO capability in such priorart.

In contrast, the processor 91 in each single-board head 64 can beprogrammed to provide CFO functions such as described above, such thattwo or more single-board heads 64 within a single chassis cancommunicate with each other to provide CFO capability. In certainembodiments, the two or more single-board heads 64 communicate with eachother only via the passive backplane 51, using Gigabit Ethernetprotocol. Among other advantages, this type of interconnectioneliminates the need for cables to connect the heads 64 to each other andto other components within the chassis. Note that in other embodiments,protocols other than Ethernet may be used for communication between theheads 64.

Thus, a method and apparatus for converting a JBOD disk drive storageenclosure into a standalone network storage system and vice versa havebeen described. Although the present invention has been described withreference to specific exemplary embodiments, it will be recognized thatthe invention is not limited to the embodiments described, but can bepracticed with modification and alteration within the spirit and scopeof the appended claims. Accordingly, the specification and drawings areto be regarded in an illustrative sense rather than a restrictive sense.

1. A method comprising: accessing an interior space of a standalone storage server that includes a chassis and a plurality of disk drives within the chassis; uninstalling at least one internal storage server head from the standalone storage server, the at least one internal storage server head configured to control access to the plurality of disk drives by an external client; and installing an input/output (I/O) module in the chassis, in place of the at least one internal storage server head, the I/O module configured to provide an interface between the plurality of disk drives and a storage server head external to the chassis.
 2. A method comprising: uninstalling an internal storage server head from within a standalone storage system, the standalone storage server including a chassis, a plurality of disk drives within the chassis, and, prior to said uninstalling, the internal storage server head within the chassis, the internal storage server head configured to control access to the plurality of disk drives by an external client; and installing an input/output (I/O) module in the chassis, in a space previously occupied by the internal storage server head, the I/O module configured to operate as an interface between the plurality of disk drives and a storage server head external to the chassis.
 3. The method of claim 2, further comprising connecting a modular storage server head to control the plurality of disk drives through the I/O module, the modular storage server head being external to the chassis when connected to the plurality of disk drives.
 4. The method of claim 3, wherein the internal storage server head is implemented on a single circuit board.
 5. The method of claim 3, wherein the standalone storage system further comprises a passive backplane within the chassis to couple the internal storage server head to the plurality of disk drives while the internal storage server head is installed, and wherein said installing the I/O module comprises coupling the I/O module to the passive backplane.
 6. The method in of claim 3, wherein the I/O module includes a loop resiliency circuit.
 7. A method comprising: accessing an internal space of a storage server, the storage server comprising a chassis and, installed within the chassis, a passive backplane, a plurality of disk drives coupled to the passive backplane, and an internal storage server head implemented on a single circuit board and coupled to the passive backplane, the internal storage server head configured to control access to the plurality of disk drives by an external client; disconnecting the internal storage server head from the passive backplane; removing the internal storage server head from the standalone storage server; installing an input/output (I/O) module in the chassis, in a space previously occupied by the internal storage server head, so that the I/O module is coupled to the passive backplane, the I/O module being configured to operate as an interface between an external storage server head and the plurality of disk drives; and connecting a modular storage server head to control the plurality of disk drives through the I/O module, the modular storage server head being external to the chassis when connected to the plurality of disk drives.
 8. The method of claim 7, wherein the I/O module includes a loop resiliency circuit. 